This invention relates in general to the field of couplers, and in particular to four-port quadrature couplers.
Four-port quadrature couplers are an important building block for many electrical components. They are an essential sub-circuit in many components such as mixers, phase detectors, balanced amplifiers, and vector modulators used in many commercial and military electrical systems.
Quadrature couplers have been implemented in various technologies based on a number of designs and topologies. At microwave and UHF frequencies above one GigaHertz (1 GHz) most quadrature coupler topologies are based on distributed elements. Two common topologies are the branch line coupler and coupled transmission line sections. However, quadrature couplers formed using distributed elements require relatively large circuit area especially at lower microwave frequencies on the order of 1-10 GHz. In coupled transmission line form, for example, each transmission line of the quadrature coupler is one-quarter wavelength long at the center operating frequency for the device. Thus, the size of the circuits is directly proportional to wavelength and inversely proportional to frequency, whereby circuits operating at lower frequencies require larger area.
Because a traditional method such as transmission line structure may require a length on the order of one inch (1 in.) at a center frequency of one GigaHertz (1 GHz), and because monolithic microwave integrated circuits (MMICs) often are only one-tenth of an inch (0.1 in.) or smaller on a side, the transmission line approach to making a quadrature coupler on MMICs is impractical for frequencies of 10 GHz or lower. It is an extreme cost disadvantage to add gallium arsenide substrate to create a structure of sufficient length to use the transmission line topology at such frequencies. And, because branch line methods rely on uncoupled transmission lines requiring an even greater circuit area, they are even less practical at frequencies below 10 GHz. Microwave circuits on MMIC below 7 or 8 GHz often do without quadrature couplers entirely.
Another approach to construction of quadrature couplers is to use lumped element topologies, using inductors and capacitors. While lumped element circuits are suitable for MMIC application, they are often limited to one input and two output (i.e., three-port) configurations. Lumped element quadrature couplers therefore typically do not fulfill the need of a four-port quadrature coupler for MMICs.
An additional shortcoming of quadrature couplers is often limited bandwidth. While some lumped element four-port couplers are possible, they exhibit narrow bandwidth. The coupler in U.S. Pat. No. 4,893,098, "90 Degree Broadband MMIC Hybrid", by Seely and Staudinger, issued on Jan. 9, 1990 to the same assignee as the present application, although suitable for MMIC application, is limited to bandwidths of less than an octave.
Thus, what is needed is a method for providing a four-port quadrature coupler which can exhibit wide bandwidth exceeding an octave and which is suitable for monolithic implementation at frequencies less than 10 GHz.